dart2js cps: Use a strictness flag on Branch instead of rewriting.

pkg/compiler/lib/src/cps_ir/cps_fragment.dart

Index: pkg/compiler/lib/src/cps_ir/cps_fragment.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_fragment.dart b/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
index 9bdb9ea15f882f2e4078ca1b9748d7841c50ec39..a9190ddf77aea155692d8abd5577be719ce2a22e 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
@@ -33,19 +33,19 @@ import '../elements/elements.dart';
 ///
 /// If `condition` is true then invoke `cont1`, else `cont2`.
 ///
-///   cps.ifTrue(condition).invokeContinuation(cont1, []);
+///   cps.ifTruthy(condition).invokeContinuation(cont1, []);
 ///   cps.invokeContinuation(cont2, []);
 ///
 /// If `condition` is true then invoke `cont` with a bound primitive:
 ///
-///   CpsFragment branch = cps.ifTrue(condition);
+///   CpsFragment branch = cps.ifTruthy(condition);
 ///   branch.invokeContinuation(cont, [branch.letPrim(arg)]);
 ///
 /// Loop and call a method until it returns false:
 ///
 ///   Continuation loop = cps.beginLoop();
 ///   var result = cps.invokeMethod(receiver, selector, ...);
-///   cps.ifFalse(result).invokeContinuation(exit, []);
+///   cps.ifFalsy(result).invokeContinuation(exit, []);
 ///   cps.continueLoop(loop);
 ///
 class CpsFragment {
@@ -198,11 +198,11 @@ class CpsFragment {
   /// Returns a new fragment for the 'then' branch.
   ///
   /// The 'else' branch becomes the new hole.
-  CpsFragment ifTrue(Primitive condition) {
+  CpsFragment ifTruthy(Primitive condition) {
     Continuation trueCont = new Continuation(<Parameter>[]);
     Continuation falseCont = new Continuation(<Parameter>[]);
     put(new LetCont.two(trueCont, falseCont,
-            new Branch(new IsTrue(condition), trueCont, falseCont)));
+            new Branch.loose(condition, trueCont, falseCont)));
     context = falseCont;
     return new CpsFragment(sourceInformation, trueCont);
   }
@@ -212,11 +212,11 @@ class CpsFragment {
   /// Returns a new fragment for the 'else' branch.
   ///
   /// The 'then' branch becomes the new hole.
-  CpsFragment ifFalse(Primitive condition) {
+  CpsFragment ifFalsy(Primitive condition) {
     Continuation trueCont = new Continuation(<Parameter>[]);
     Continuation falseCont = new Continuation(<Parameter>[]);
     put(new LetCont.two(trueCont, falseCont,
-            new Branch(new IsTrue(condition), trueCont, falseCont)));
+            new Branch.loose(condition, trueCont, falseCont)));
     context = trueCont;
     return new CpsFragment(sourceInformation, falseCont);
   }